By process of elimination, the first one, because I watched it a ridiculous amount of times as a kid... that said, I haven't seen it in years, so it's not like I know it like the back of my hand, either =P
Saw Toy Story 2 once or twice, don't remember much except I guess they expanded on Woody's deal a bit more. Never saw 3, heard mixed things about it. Who knows what 4 is gonna be like.